Read-out control for use with a domain expansion recording medium

ABSTRACT

The present invention relates to a method and apparatus for controlling a read-out operation from a magneto-optical recording medium, comprising a storage layer and a read-out layer, wherein an expanded domain leading to a reading pulse is generated in the read-out layer by copying a mark region from the storage layer to the read-out layer upon heating by a radiation power and with the aid of the external magnetic field. A switching time of the external magnetic field is derived from the reading pulse and a space run-length is determined on the basis of a time delay between the switching time and the reading pulse. The delay time need not have a fixed period such that space increments smaller than the channel bit length can be used for space run length coding. This results in a significantly improved resolution.

The present invention relates to a method, apparatus and record carrierfor controlling read-out from a magneto-optical recording medium, suchas a MAMMOS (Magnetic AMplifying Magneto-Optical System) disk,comprising a recording or storage layer and an expansion or read-outlayer.

In magneto-optical storage systems, the minimum width of the recordedmarks is determined by the diffraction limit, that is by the NumericalAperture (NA) of the focussing lens and the laser wavelength. Areduction of the width is generally based on shorter wavelength lasersand higher NA focussing optics. During magneto-optical recording, theminimum bit length can be reduced to below the optical diffraction limitby using Laser Pulsed Magnetic Field Modulation (LP-MFM). In LP-MFM, thebit transitions are determined by the switching of the field and thetemperature gradient induced by the switching of the laser. For read-outof the small crescent-shaped marks recorded in this way, Magnetic SuperResolution (MSR) or Domain Expansion (DomEx) methods have been proposed.These technologies are based on recording media with severalmagneto-static or exchange-coupled RE-TM layers. According to MSR, aread-out layer on a magneto-optical disk is arranged to mask adjacentbits during reading, while, according to domain expansion, a domain inthe center of a spot is expanded. The advantage of the domain expansiontechnique over MSR results in that bits with a length below thediffraction limit can be detected with a similar signal-to-noise ratio(SNR) as bits with a size comparable to the diffraction limited spot.MAMMOS is a domain expansion method based on magneto-statically coupledstorage and read-out layers, wherein a magnetic field modulation is usedfor expansion and collapse of expanded domains in the read-out layer.

In the above-mentioned domain expansion techniques, like MAMMOS, awritten mark from the storage layer is copied to the read-out layer uponlaser heating with the aid of an external magnetic field. Due to the lowcoercivity of this read-out layer, the copied mark will expand to fillthe optical spot and can be detected with a saturated signal level whichis independent of the mark size. Reversal of the external magnetic fieldcollapses the expanded domain. A space in the storage layer, on theother hand, will not be copied and no expansion occurs. Therefore, nosignal will be detected in this case.

The laser power used in the read-out process should be high enough toenable copying. On the other hand, a higher laser power also increasesthe overlap of the temperature-induced coercivity profile and the strayfield profile of the bit pattern. The coercivity H_(c) decreases and thestray field increases with increasing temperature. When this overlapbecomes too large, correct read-out of a space is no longer possible dueto false signals generated by neighboring marks. The difference betweenthis maximum and the minimum laser power determines the power margin,which decreases strongly with decreasing bit length. Experiments haveshown that with the current methods, bit lengths of 0.10 μm can becorrectly detected, but at a power margin of virtually nothing (1 bit ofa 16 bit DAC). Thus, for highest densities the power margin remainsquite small so that optical power control during read-out is essential.

In conventional MAMMOS read-out, the external magnetic field ismodulated with a period corresponding to the size of a channel bit.Thus, a bit decision is made for each channel bit (mark or space, i.e.up or down magnetization). However, synchronization of the externalfield modulation with the bit pattern on the disc is critical. Forexample, when the copy window is close to its maximum size for correctread-out, a small phase error already introduces a false peak. For thissynchronization, timing fields and/or a wobble in the track can be used.In this way, quite reasonable frequency control is possible, but phaseerrors are very difficult to avoid.

It is an object of the present invention to provide a method, apparatusand record carrier for domain expansion read-out control with improvedsynchronization and resolution or storage density.

This object is achieved by a method as claimed in claim 1 or 14, by anapparatus as claimed in claim 6 or 15, and by a record carrier asclaimed in claim 12.

Accordingly, the delay time determined does not have a fixed period suchthat space increments smaller than the channel bit length can be usedfor space run-length coding. This results in a significantly improvedresolution without additional demands on the field coil of the magnetichead and its driver.

Preferably, the time delay is measured between a field reversal to theexpansion direction and the rising edge of said reading pulse.

A pulse correction has to be performed in a mark run-length detectionbased on said reading pulse. Thus, decoding errors due to additionalfalse peaks achieved in the data-dependent field switching can beprevented.

The deriving means may be arranged to derive said switching time from adetected rising edge of said reading pulse. Then, the deriving means maybe arranged to set the time period between the detection and theswitching time in accordance with a channel bit period of said markregion.

The determination means may comprise a timer means for counting the timedelay. Additionally, the determination means may be arranged todetermine a shift in the switching time.

Other advantageous further developments are defined in the dependentclaims.

The present invention will be described hereinafter on the basis of apreferred embodiment and with reference to the accompanying drawings, inwhich:

FIG. 1 shows a diagram of a magneto-optical disk player according to apreferred embodiment,

FIG. 2 shows read-out waveforms for a copy window size equal to half ofthe channel bit length,

FIG. 3 shows read-out waveforms for a copy window size between half ofand a full channel bit length,

FIG. 4 shows read-out waveforms for a fractional increase in spacerun-lengths,

FIGS. 5A and 5B show read-out waveforms for a 50% write strategy atdifferent copy window sizes, and

FIG. 6 shows a diagram indicating a characteristic of a MAMMOS peakdelay as a function of the space run-length.

The preferred embodiment will now be described on the basis of a MAMMOSdisk player as indicated in FIG. 1.

FIG. 1 schematically shows the construction of the disk player accordingto the preferred embodiments. The disk player comprises an opticalpick-up unit 30 having a laser light radiating section for irradiationof a magneto-optical recording medium or record carrier 10, such as amagneto-optical disk, with light that has been converted, duringrecording, to pulses with a period synchronized with code data, and amagnetic field applying section comprising a magnetic head 12 whichapplies a magnetic field in a controlled manner at the time of recordingand playback on the magneto-optical disk 10. In the optical pick-up unit30 a laser is connected to a laser driving circuit which receivesrecording and read-out pulses from a recording/read-out pulse adjustingunit 32 to thereby control the pulse amplitude and timing of the laserof the optical pick-up unit 30 during a recording and read-outoperation. The recording/read-out pulse adjusting circuit 32 receives aclock signal from a clock generator 26 which may comprise a PLL (PhaseLocked Loop) circuit.

It is to be noted that for reasons of simplicity the magnetic head 12and the optical pick-up unit 30 are shown on opposite sides of the disk10 in FIG. 1. However, according to the preferred embodiment, theyshould be arranged on the same side of the disk 10.

The magnetic head 12 is connected to a head driver unit 14 and receives,at the time of recording, code-converted data via a phase adjustingcircuit 18 from a modulator 24. The modulator 24 converts inputrecording data to a prescribed code.

At the time of playback, the head driver 14 receives a timing-signal viaa playback adjusting circuit 20 from a timing circuit 34, the playbackadjusting circuit 20 generating a synchronization signal for adjustingthe timing and amplitude of pulses applied to the magnetic head 12. Thetiming circuit 34 derives its timing signal from the data read-outoperation, as described later. Thus, data-dependent field switching canbe achieved. A recording/playback switch 16 is provided for switching orselecting the respective signal to be applied to the head driver 14 atthe time of recording and at the time of playback.

Furthermore, the optical pick-up unit 30 comprises a detector fordetecting laser light reflected from the disk 10 and for generating acorresponding reading signal applied to a decoder 28 which is arrangedto decode the reading signal to generate output data. Furthermore, thereading signal generated by the optical pick-up unit 30 is applied to aclock generator 26 in which a clock signal obtained from embossed clockmarks of the disk 10 is extracted, and which applies the clock signalfor synchronization purposes to the recording pulse adjusting circuit 32and the modulator 24. In particular, a data channel clock may begenerated in the PLL circuit of the clock generator 26. It is to benoted that the clock signal obtained from the clock generator 26 may aswell be applied to the playback adjusting circuit 20 to thereby providea reference or fallback synchronization which may support the datadependent switching or synchronization controlled by the timing circuit34.

In the case of data recording, the laser of the optical pick-up unit 30is modulated with a fixed frequency, corresponding to the period of thedata channel clock, and the data recording area or spot of the rotatingdisk 10 is locally heated at equal distances. Additionally, the datachannel clock output by the clock generator 26 controls the modulator 24to generate a data signal with the standard clock period. The recordingdata are modulated and code-converted by the modulator 24 to obtainbinary run-length information corresponding to the information of therecording data.

The structure of the magneto-optical recording medium 10 may, forexample, correspond to the structure described in the JP-A-2000-260079.

In the preferred embodiment shown in FIG. 1, the timing circuit 34 isprovided for applying a data-dependent timing signal to the playbackadjusting circuit 20. As an alternative, the data-dependent switching ofthe external magnetic field may as well be achieved by applying thetiming signal to the head driver 14 so as to adjust the timing or phaseof the external magnetic field.

According to the preferred embodiment, timing information is obtainedfrom the (user) data on the disc 10. To achieve this, the playbackadjusting circuit 20 or the head driver 14 are adapted to provide anexternal magnetic field which extends normally in the expansiondirection. When a rising signal edge of a MAMMOS peak is observed by thetiming circuit 34 at an input line connected to the output of theoptical pick-up unit 30, the timing signal is applied to the playbackadjusting circuit 20 such that the head driver 14 is controlled toreverse the magnetic field after a short time so as to collapse theexpanded domain in the read-out layer and shortly after that reset themagnetic field to the expansion direction. The total time between thepeak detection and the field reset is set by the timing circuit 34 tocorrespond to one channel bit length on the disk 10 (times the lineardisc velocity).

With the data-dependent field switching method mentioned above,synchronization is no longer required during read-out as the switchingtime is derived directly from the data. Moreover, the derived switchingtimes can be used to further advantage as input for the PLL circuit ofthe clock generator 26 to provide an accurate data clock. More precisedata recovery, based on the space run-length information in the timedelay, can thus be obtained.

FIGS. 2 to 5B each show diagrams indicating from top to bottom a storagelayer with its mark and space regions (indicated by upward and downwardarrows, respectively) and with a copy window size w indicating thespatial width of the copy operation, and waveforms of an overlap signal,the alternating external magnetic field and the MAMMOS read-out signal,respectively. The overlap signal indicates a time-dependent value of theoverlap between the coercivity profile and the stray field, which leadsto a MAMMOS signal or peak when an external magnetic field is applied.In particular, a MAMMOS peak will be generated during the time period ofthe positive external magnetic field. Due to the fact that the overlapsignal may extend until a neighboring (previous or next) positive periodof the external magnetic field, additional peaks can be generated in theMAMMOS signal.

In FIG. 2, each mark run-length (indicated by upward arrows) will giveone more MAMMOS peak (hatched) than its length divided by the channelbit length b which corresponds to one section in the schematically shownstorage layer. Thus, an I1 mark run-length (length b) will give twopeaks instead of one, an I2 mark run-length (length 2 b) will give threepeaks instead of two, etc. A comparison with FIG. 3, where correspondingwaveforms are illustrated for a larger size w of the copy window,reveals that this situation remains valid for 0w<b. Thus, acorresponding correction algorithm has to be applied in the decoder 28to obtain the mark run-length and hence the correct output data DO.

The space run-lengths in this scheme are derived from the time that themagnetic field extends in the expansion direction (positive values)before the next MAMMOS peak appears. These times d, d1, d2 are indicatedin the FIGS. 2 to 5B. When a rising signal edge of the magnetic field isobserved by the timing circuit 34, for example, on the basis of theoutput signal of the head driver 14, a timer circuit or timer functionprovided in the timing circuit 34 is started which counts the time untila rising signal edge of the next MAMMOS peak is detected at the outputof the optical pick-up circuit 30.

It will be clear that a space run-length equal to the channel bit lengthb has no delay (no bold line) so that it cannot be detected. In FIG. 2,a delay d corresponding to an −I2 space run length (length 2 b, “−”indicates a space) is indicated.

In FIG. 3, delays d1 and d2 corresponding to a −I2 and a −I3 spacerun-length, respectively, are indicated for a larger copy window size w.Furthermore, in FIG. 4, delays d1 and d2 are shown for a fractionallyincreased −I1.5 space run-length and a −I3 space run-length at the samecopy window size w used in FIG. 3.

FIG. 6 shows a diagram indicating a characteristic curve of the peakdelay d as a function of the space run-length SRL. From the FIGS. 3 and4 it can be gathered that the delay determined at the timing circuit 34is a smooth function of the space run-length. Therefore, there is noreason to increment space run-lengths by b (indicated by dashed gridlines in FIG. 6) as in the case of mark run-lengths. If the jitter inthe read-out signal is small enough, increments (much) smaller than b(dotted grid lines in FIG. 6) can be used, for example, in the modulator24, thus significantly increasing the storage density. The delay ddetermined can be applied from the timing circuit 34 to the decoder 28such that a correct or precise decoding function for the spacerun-lengths can be achieved.

Furthermore, an improved write strategy may be necessary to avoidmissing peaks in long mark run-lengths; it is also useful to increasethe storage density and/or the power margin. In particular each markchannel bit (length b) may be composed of a small mark and a small space(total length b, mark portion as small as possible) as is illustrated inFIG. 5A. This effectively reduces the overlap and thus allows largervalues of the copy window size w. For a 50% write strategy (length of amark portion equals. the length of a space portion within the composedmark region), as is sketched in FIGS. 5A and 5B, the maximum copy windowsize w is 1.5b instead of b (to keep one additional peak per markrun-length, the minimum copy window size w becomes b/2). A larger copywindow is very advantageous as this reduces the demands on the laserpower control (or allows higher densities).

The invention offers a solution also if the power control that can beattained in the recording system is not sufficient and larger copywindows are required. In that case the minimum space run-length willhave to be increased. For example, a space run-length larger than 2bwill allow a maximum window of 2b for conventional writing and a windowof 2.5b for a 50% write strategy. The storage density will now decreasedue to a reduction of the code rate. The minimum mark run-length canremain at b, that is, an I1 mark run-length can be used.

The present invention can be applied to any reading system for domainexpansion magneto-optical disk storage systems. Any waveformcharacteristic of the read-out signal, which indicates a change in theread-out signal, can be used in the analysis. The function of the timingcircuit 34 may be provided by a discrete hardware unit or,alternatively, by a corresponding control program controlling a moregeneral processing unit. The preferred embodiments may thus vary withinthe scope of the attached claims.

1. A method of controlling a read-out operation from a magneto-opticalrecording medium (10), said recording medium comprising a storage layerand a read-out layer, wherein an expanded domain leading to a readingpulse is generated in said read-out layer by copying a mark region fromsaid storage layer to said read-out layer upon heating by a radiationpower and with the aid of an external magnetic field, said methodcomprising a step for deriving a switching time of said externalmagnetic field from said reading pulse, and a determination step fordetermining a space run-length on the basis of a time delay between saidswitching time and said reading pulse.
 2. A method according to claim 1,wherein said time delay is the time between a field reversal to theexpansion direction and the rising edge of said reading pulse.
 3. Amethod according to claim 1, wherein the time period from said readingpulse to a reset of said external magnetic field is set to correspond toone channel bit length.
 4. A method according to claim 1, wherein apulse correction is performed in a mark run length detection based onsaid reading pulse.
 5. A method according to claim 1, wherein spaceincrements smaller than a channel bit length are detected in saiddetermination step.
 6. A reading apparatus for controlling a read-outoperation from a magneto-optical recording medium (10), said recordingmedium comprising a storage layer and a read-out layer, wherein anexpanded domain leading to a reading pulse is generated in said read-outlayer by copying a mark region from said storage layer to said read-outlayer upon heating by a radiation power and with the aid of an externalmagnetic field, said apparatus comprising deriving means (34, 20) forderiving a switching time of said external magnetic field from saidreading pulse, and determination means (34) for determining a space runlength based on a time delay between said switching time and saidreading pulse.
 7. A reading apparatus according to claim 6, wherein saidderiving means (34, 20) is arranged to derive said switching time from adetected rising edge of said reading pulse.
 8. A reading apparatusaccording to claim 6, wherein said deriving means (34, 20) is arrangedto set the time period between said detection and said switching time inaccordance with a channel bit period of said mark region.
 9. A readingapparatus according to claim 6, wherein said determination means (34)comprises a timer means for counting said time delay.
 10. A readingapparatus according to claim 6, wherein said determination means (34) isarranged to determine a shift in said switching time, and to apply theresult of said determination to a copy window control means (30).
 11. Anapparatus according to claim 6, wherein said reading apparatus is a diskplayer for MAMMOS disks.
 12. A magneto-optical record carrier comprisinga storage layer and a read-out layer, wherein an expanded domain leadingto a pulse in a reading signal is generated in said read-out layer bycopying a mark region from said storage layer to said read-out layerupon radiation heating and with aid of an external magnetic field, saidrecord carrier (10) comprising space increments smaller than a channelbit length of said mark region.
 13. A record carrier according to claim12, wherein said record carrier is a MAMMOS disk (10).
 14. A recordingmethod for recording information on a magneto-optical recording medium(10), said recording medium comprising a storage layer and a read-outlayer, said method comprising the steps of recording said information bymodulating run lengths of mark and space regions in said storage layer,and performing said run length modulation of space regions by usingincrements smaller than a channel bit length of said mark regions.
 15. Arecording apparatus for recording information on a magneto-opticalrecording medium (10), said recording medium comprising a storage layerand a read-out layer, said apparatus comprising recording means (12, 30)for recording said information by modulating run lengths of mark andspace regions in said storage layer, and means (24) for performing saidrun length modulation of space regions by using increments smaller thana channel bit length of said mark regions.